Even though electrochemistry belongs to the first chemical techniques, synthetic chemists did not really take to it. But thanks to variety of social, economic and technical developments, that is about to change. ‘Now that we are generating more and more green electricity, we see that electrons become abundantly available’, says Adri Minnaard, professor in Organic Chemistry at the University of Groningen. ‘It would be a shame not to use these electrons in synthesis.’

That is why Minnaard together with his Groningen-based colleague associate professor Martin Witte, set up a project within the ARC CBBC consortium to investigate the potential of electrosynthesis. Minnaard: ‘We want to use this method to convert carbohydrates to other useful molecules.’ Their choice for using carbohydrates gives them a unique position within the consortium. ‘Other groups look at different starting materials, but I think carbohydrates are very interesting structures that are widely available as sustainable starting materials.’

‘It would be a shame not to use these electrons in synthesis’

Adri Minnaard

According to Minnaard, the combination of electrosynthesis and carbohydrates can also be very interesting to industry. ‘There are several companies that work with electrolysers or other electrochemical setups. It takes some planning to build such a factory, but once you have it in place, it works very well.’

Oxidizing sugars

But there are still plenty of obstacles to overcome. The first step the team is trying to realize is to oxidize sugars. However, this turned out to be quite an ambitious target. ‘Carbohydrates are only soluble in water or other very polar solvents’, explains Imke Bartels, PhD student on the project. ‘This makes for a system with a lot of OH-groups that can almost all be oxidized. That forced us to find a way to selectively react only those groups that we want to change.’ A complicating factor is that nobody really knows how these electrochemical synthesis reactions occur. Minnaard: ‘We can think of mechanisms and make reasonable assumptions, but we can’t look at the surface of the electrodes to see what actually happens.’

‘We are just beginning to learn the secrets of these reactions’

Imke Bartels

And Bartels knows from experience that the carbohydrates often don’t behave as expected. ‘We had some surprising results, even with reactions that are thoroughly described in the literature. It is a real challenge to figure out what has happened.’ Luckily for Bartels, there are other groups that work on electrosynthesis, and she was able to spend time in the labs of more experienced researchers to get the hang of things. ‘I just spend six weeks in the group of Shannon Stahl at the University of Wisconsin. I was very grateful they could help me. They have lots of experience with electrochemical oxidation, so I was able to really take steps in the right direction.’

Build your own set-up

With the overall increasing interest in electrosynthesis, more progress becomes tangible. ‘Until recently, there was almost no equipment available, meaning that you always had to build your own set-up’, Bartels says. ‘This surely had advantages as well, because you specifically adjust everything to your needs, but it also makes the research less reproduceable.’ Now, several companies offer a standardized apparatus for electrosynthesis. ‘It is basically a stirring plate, but it comes with a holding device where you can just click in your vial. The vial has a special lid that can be fitted with different kinds of electrodes. Just add your reagents and you’re good to go.’ By combining different set-ups, Bartels is now able to attach ketones and nitrile moieties on the sugars, and that paves the way to other structures. ‘We don’t have a specific molecule in mind that we want to make’, says Minnaard. ‘But we are exploring different opportunities and trying to understand the process along the way.’

Crosslinker

Of course, they have ideas for structures that would be nice to develop. Minnaard: ‘In another ARC CBBC project, we modified dextran to create a sustainable crosslinker for coatings. BASF was interested, but we have to use palladium catalysis to obtain the product and that is not suitable for large scale production. Thus, it would be interesting to make the same product with this electrosynthesis technique.’ And there may emerge more of these applications in the future, but for now, Bartels’ priority is on trying to understand the process. ‘We are just beginning to learn the secrets of these reactions, and there is still a lot we don’t know’, she says. Minnaard is confident they will get there in the end. ‘We are taking very small steps, but they are steps in the right direction. We could have also taken a model compound that is easier, but we are using carbohydrates from the start. This makes it harder to solve and it will take a while, but if we manage to figure this out, we are immediately where we want to be.’